Conventionally, as a monitoring apparatus using an imaging device for capturing an image of a welding portion, an apparatus has been publicly known in which the dynamic range of the imaging device is increased to allow the imaging device to accurately capture an image of a wide luminance range from a high-luminance portion near molten metal to a portion on the periphery of the welding portion when capturing an image of the welding portion and its periphery in arc welding, thereby displaying a high-definition image in which the molten metal and its periphery having a high luminance and a dark portion having a low luminance are both clearly shown (see, for example, Patent Document 1).